water cycle, or hydrologic cycle, is the continuous circulation of
water and water vapor between the earth and the atmosphere. It is
driven by heat and energy from the sun. Water vapor rises from oceans,
lakes, rivers, forests, fields, plants and animals by the process
of evaporation. The evaporated water is carried into the atmosphere
where it cools and develops into visible moisture as clouds and fog
(condensation). It falls back to earth as rain or snow (precipitation),
completing the cycle.
precipitation falls on the surface of the earth, the water may quickly
run into streams and eventually be carried to the sea. It may also
be held in lakes or oceans, sometimes for many years. Or it may
soak into the ground where it might be held for a few years to thousands
of years, depending on how deep it goes into the earth. Much of
the precipitation that falls is evaporated again almost immediately.
Eventually the water that is held in oceans, lakes and the ground
again becomes part of the cycle.
you consider the earth as a whole, the water cycle is a "closed"
cycle; that is, no water is gained or lost. But if you consider
a particular location on earth, you may see "wet" years
and "dry" years when there is more or less rain than usual.
In the polar regions, there may be accumulation or loss of ice and
snow, depending on the earth's temperature. Long-term shifts in
the earth's temperature has led to ice ages in the past; at present
the earth is in a warming trend, with ice and snow in polar regions
and in glaciers being transformed to liquid water in the earth's
oceans. If this trend continues, coastal areas could be threatened
by sea level rise.
resource for further information can be found at http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hyd/home.rxml